In radio frequency tuners including quadrature frequency converters, it is necessary to reduce or eliminate imbalances between the in-phase (I) and quadrature (Q) signal paths within the converter in order to achieve acceptable performance. Imbalances in phase and gain in the I and Q signal paths following manufacture are inevitable because of component value tolerances and it is therefore necessary to perform an alignment procedure in order to reduce such balances to acceptable levels. However, performing such alignment during manufacture increases the cost. Also, drifting in component values caused by temperature and ageing effects results in imbalance which alignment during manufacture cannot address.
U.S. Pat. No. 6,009,317 discloses a “complex signal receiver” having an arrangement for reducing phase and gain imbalance. Imbalance is measured and the relative phases and gains of the I and Q channels are changed iteratively so as to reduce imbalance.
U.S. Pat. No. 6,714,776 discloses a single conversion receiver which provides image rejection using I and Q signal paths. Peak detectors receive the I and Q signals and the difference between the detector outputs is used to control gain imbalance compensation.
GB 2 390 242 discloses an arrangement which compensates for non-linearities in a mixer of a direct-conversion receiver. The mixer has a “parabolic” term in its transfer characteristic and this gives rise to second harmonic distortion. The input signal to the mixer is supplied to a pre-distortion circuit, which squares the signal to as to add distortion which at least partly cancels the mixer second harmonic distortion.